If you assume a mortgage without qualification you risk the bank calling the loan due as they could deem the loan in default. They may do it, but often do not. This risk increase if the interest rate spread on the assumed mortgage and current mortgage rates is high, i.e. if the bank could make more money getting a new loan.
The benefit might be lower interest rates of the assumed mortgage than current market rates.
The drawback might be higher interest rates of the assumed mortgage than current market rates.
Another benefit [mainly to seller] is no discharge penalty.
A 3rd benefit [to buyer] is access to a mortgage that s/he couldn't get on new qualification.
btw: is this mortgage CMHC insured ? If assumed, the seller is still at risk, unless his obligation is discharged in writing.